1. Field of the Invention
The present invention relates to the preparation of dosages of tracer materials for use in diagnostic nuclear medicine.
2. Description of the Prior Art
Nuclear medicine has become an extremely valuable tool for diagnosing medical ailments. Abnormalities in the soft tissue of the organs of the body can be detected in a non-invasive fashion. Diagnostic nuclear medicine typically involves the detection of a tracer quantity of an injected substance containing a known, low-level dosage of a radionuclide.
The chemical composition of the radionuclide is such that it is compatible with carrier materials and with the body chemistry of patients. The molecules containing the radionuclides are attached to molecules of a carrier material which is chosen to exhibit an affinity for migration toward particular areas of the human body. For example, radionuclide carriers are commercially available which, when injected into a patient, will tend to migrate toward the patient's bones, heart, brain, kidneys, gall bladder, lungs, and liver. Migration of the carrier toward the selected region of the body occurs through the circulatory system of the patient.
Circulatory abnormalities in particular portions of the body are detected by comparing the circulation of the carrier and radionuclide in a particular patient with the corresponding circulation in patients known to be normal and in patients known to be abnormal in certain regards.
As the carrier and radioactive dosage are carried through a patient's circulatory system, some of the radionuclides will decay, thus emitting detectable rays, such as gamma rays. Gamma rays pass through the body of a patient and can be detected by such devices as scintillation detectors. Scintillation detectors are typically formed with sodium iodide crystals which emit flashes of light when subjected to gamma radiation. The locations of the flashes of light of the decaying radionuclides are ascertained through one or more devices such as photomultiplier tubes. Photomultiplier tubes convert visible flashes of light into electrical pulses.
While several different radionuclides exist which can be employed in nuclear medicine, the most widely used substance is technetium - 99m. Technetium - 99m is the tracing material of choice in the vast majority of diagnostic tests in the field of nuclear medicine. Technetium - 99m has a half life of only six hours so that the level of radioactivity in a tracer dose administered to a patient decays rapidly. Also, because the half life of technetium - 99m is so short a relatively low dosage of the substance will produce a significant number of nuclear events which can be detected to analyze any abnormality in the patient. Also, the rapid decay of the tracer quantity of material poses a minimum health hazard to the patient and allows a fairly rapid sequence of tests utilizing radioactive substances to be performed without interference from remnant traces of prior dosages from earlier tests.
Technetium - 99m will attach readily to a number of different carrier compounds which will selectively migrate toward locations of interest in the body of the patient to which the carrier is administered. For example, stannous pyrophosphate will migrate toward the heart and the bones of a patient. Stannous methylene diphosphonate, when injected, will tend to collect in the bones of a patient. Stannous glucoheptonate migrates toward the brain and kidneys of a patient. Stannous HIDA shows an affinity for a patient's gall bladder. Stannous macroaggregated albumin tends to collect in the lungs of a patient. Stannous microaggregated albumin migrates toward the patient's liver. Stannous DTPA tends to migrate towards a patient's brain and kidneys. Stannous dimercaptosuccinic acid shows an affinity toward a patient's kidneys.
All of the foregoing materials which exhibit a particular affinity for the organs or regions of the body indicated are normally stored in lyophilized form. That is, the substances are freeze-dried and can be reconstituted with liquids. When mixed with a saline solution containing sodium technetium pertechnetate the foregoing tracer compounds will dissolve almost instantly and combine with the technetium pertechnetate. The mixture can then be used as a tracer material useful in diagnostic tests in the field of nuclear medicine.
According to conventional practice, patients are scheduled for particular diagnostic tests which employ radionuclides and carrier materials, such as those specified, which show an affinity towards specific portions of the body of a living subject. According to conventional practice, a batch of a tracing substance is prepared each morning in the nuclear medicine laboratory of a hospital for each different test scheduled to be administered during the course of the day. The technetium pertechnetate substance is produced and stored in a lead-lined container known as a "pig". The "pigs" are labeled according to the stannous compound which is to be introduced therein. A saline solution of sodium technetium pertechnetate having a radioactivity level of typically between 200 and 300 millicuries is introduced into each "pig". A quantity of a lyophilized stannous compound for each test to be performed during the course of the day is mixed into the saline pertechnetate solution in each "pig". Separate dosages are withdrawn from the appropriate "pigs" as required, and are injected into the patient.
Several problems exist in connection with the present method of preparing doses of a tracing substance for use in nuclear medicine. Specifically, as the radionuclide compound sits in the "pig" throughout the course of the day, it tends to separate from the carrier chemical. Accordingly, the level of radioactive material carried to the organ or portion of the body of interest by a dosage drawn late in the day is likely to be less than the corresponding level for a test performed early in the morning. Also, as the liquid mixtures are prepared in each "pig" for the different diagnostic tests involved, relatively large quantities of the technetium tracer are committed for use with particular carrier materials. That is, for example, if a number of bone scans are scheduled for a particular day, the technician in the nuclear medicine laboratory will mix a quantity of stannous pyrophosphate with an amount of technetium pertechnetate more than sufficient to perform the number of bone scans scheduled for that day. However, if one of the bone scans is cancelled and an unscheduled lung scan is to be performed, an entirely new batch of tracing substance must be prepared. Consequently, the technician is exposed to an inordinately great level of radiation since each batch of tracing substance which is mixed will have a level of radioactivity of between about 200 and 300 millicuries. The technician is therefore exposed to the level of radiation necessary to prepare batches of tracing material for all of the tests which are to be performed during the day.
A further problem which arises in the conventional method of preparing doses of a tracing substance is that dosages of a radioactive tracing substance will sometimes be drawn from the wrong "pig". Errors of this type frequently occur due to labeling errors on the "pig", different lyophilized carriers are introduced into the "pigs" on different days. When a scan is performed of a patient to whom a dosage with an incorrect carrier has been administered, the test results are meaningless and the test must be repeated.
A further problem with the conventional practice is that much of the radionuclide dosage prepared is wasted. For example, a saline solution of sodium technetium pertechnetate having an initial radioactivity level of 300 millicuries is normally introduced into each "pig" in the morning. However, during the course of the day, perhaps only one or two doses of the tracing substance may be drawn from each different "pig". Since each dose to be injected into a patient has a radioactivity level of about 25 millicuries, technicians in nuclear medicine laboratories are subjected to inordinately large levels of radioactivity. Furthermore, once the level of radioactivity has decayed to where the dosages are no longer usable, disposal of the unused radioactive material becomes a problem.